This came up in a talk given by a museum curator, he said for his catalog descriptions flat art is quoted by the actual image size without allowing for any area the matte or frame might cover. So by that rationale 15x23 is right for your 16 x 24 with the 1/2" white borders. I have once in a while seen something like "to fit a 16 x 20 frame" in a print description, that would seem to allow some wiggle room on actual size, you could reasonably assume the rabbet would cover 1/4 to 1/2 all the way around and therefore the print doesn't need to be quite as big. I suppose what matters is that if you quote a size bigger than the actual image size, you need to add some qualification in your description.
In the case where a paper matte is to cover part of the image, usually about 1/8" to 1/4" of the border of the image is covered, which reduces the viewable width of the print by 1/4 to 1/2". You gotta be a little brave to cut 1/8" overlap mattes, for images bigger than about 13x19 you should go for 3/16" to 1/4" to allow for any gradual sagging or shrinking of the matte and generally make like life easier. Although one feels compelled to minimize any print obscuration, nothing looks as stupid in the frame as a piece of white border peeking out from under the matte in just one area.
